The present invention relates to optical glasses, in particular glasses designed for use in high correction ophthalmic lenses.
The use of glasses having a high index of refraction, viz., about 1.70, instead of classic glasses having an index of refraction of about 1.523, makes it possible to reduce the curvatures and, consequently, to decrease the thickness of a lens. This feature leads to two advantages:
First, the lens weighs less (assuming the density of the glass is not too high, i.e., typically less than 3.2 g/cm.sup.3, which characteristic precludes the use of the classic "flint" glasses of high index containing BaO and PbO) making for greater comfort for the spectacle wearer; and
Second, the aesthetic appearance of the lens is considerably improved.
Nevertheless, as a general rule, increasing the refractive index of a glass leads to a decrease in the Abbe number (or an increase in the dispersion) of the glass. If the dispersion of a glass is too high, chromatic aberration ("iridescence" at the edge of a lens for example) becomes substantial. As a consequence, a high Abbe number (generally greater than 34) is sought, concurrently with a low density and high refractive index.
U.S. Pat. No. 4,057,435 discloses glasses demonstrating refractive indices of about 1.7 or higher, an Abbe number of at least 40, a density of about 3 g/cm.sup.3, and a weight loss, as measured in the AO test, of less than 5 mg/cm.sup.2. Those glasses consisted essentially, expressed in terms of weight percent on the oxide basis, of about
______________________________________ CaO 18-28 BaO 0-15 CaO + BaO 20-34 La.sub.2 O.sub.3 9-15 TiO.sub.2 9-15 MgO 0-5 Na.sub.2 O 0-4 Li.sub.2 O 0-4 Na.sub.2 O + Li.sub.2 O 0-4 Al.sub.2 O.sub.3 0-5 Gd.sub.2 O.sub.3 0-5 Y.sub.2 O.sub.3 0-5 Nb.sub.2 O.sub.5 0-5 ZrO.sub.2 0-4 WO.sub.3 0-4 ZnO 0-4 B.sub.2 O.sub.3 30-54 SiO.sub.2 remainder but .ltoreq.15 B.sub.2 O.sub.3 + SiO.sub.2 30-54 ______________________________________
As can be observed, the patented glasses satisfied the above criteria of high correction ophthalmic lenses with regard to refractive index, dispersion, and density. However, an improvement in the chemical durability of the glasses was demanded.